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Introduction

As defined by International Association for Study of Pain (IASP), pain is “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” It further states that “pain is always subjective, and each individual learns the application of the word through experiences related to injury in early life.” IASP also argues that activity induced in nociceptor and nociceptive pathways by noxious stimulus is not pain.

For people suffering from pain, their initial response is to avoid activity and seek rest. And yet exercise therapy is often prescribed as a treatment option to manage pain. There are known benefits of exercise and regular physical activity. CDC lists following as the benefits of physical activity: controls weight, reduces risk of cardiovascular disease and metabolic disease, reduces risk of some cancers, strengthens bones and muscles, improves mental health and mood, improves ability to perform daily activities and prevent falls, and increases chances of living longer. Exercises and physical activity not only have benefits in healthy individual but also has proven benefits in patients. Considering this some authors have postulated an idea that exercise should be considered as a drug[1]. And often most important benefit of exercise for patients is improved pain control[2].

Exercise Induced Hypoalgesia (EIH)

EIH is phenomenon which has been extensively studied in the literature, and results of which are ambiguous. EIH is poorly understood, and it is characterized secondary to increase in pain threshold and tolerance in addition to reduction of pain intensity rating during and after exercise[3].

Another proposed mechanism for EIH is Endocannabinoids[6]. Exercise increases serum concentrations of endocannabinoids which may contribute to control of pain transmission. Koltyn et al[5] suggested involvement of non-opioid mechanism in EIH following isometric exercise. Given this two mechanisms for EIH, the exact mechanism remains unknown.

Effects of Different Types of Exercise in Pain Management

Aerobic exercise/endurance training and resistive exercise/strength training are two different types of exercises which can be aquatic or land-based. Pain control is achieved differently with different types of exercises. Prescribing appropriate intensity and frequency of exercise is important in achieving the desired effects of hypoalgesia.

Aerobic Exercise/Endurance training

Effects of aerobic exercises on pain has been extensively studied in the literature. The intensity of the exercise should be well tolerated for the exercise to be effective. It is been said that the aerobic exercise should target larger muscle groups, involve repetitive muscle contraction and elevate resting heart rate to target heart rate for at least 20 minutes[7]. The therapeutic window for the aerobic training is extremely important, as some patients may exhibit worsening of symptoms secondary to exercise. Musculoskeletal pain is one of the side effects of exercise, and hence deciding a right balance is very important. Too little exercise is not beneficial whereas too much would aggravate the symptoms.

Various prescribed workloads have been suggested which result in exercise induced hypoalgesia. Hoffman et al[7] concluded that 30 min of treadmill exercise at 75% VO2max resulted in significant decrease of pain ratings. Whereas showed no significant changes with treadmill exercise of 10 min at 75% VO2max and 30 min at 50% VO2max. In a study done by Naugle et al[2] the effects of moderate and vigorous intensity aerobic exercise were studied on pain modulation. The subjects performed 25 min of vigorous stationary cycling at 70% of heart rate reserve (HRR) and 25 min of moderate intensity stationary cycling at 50-55% HRR. The provided evidence suggests that vigorous and moderate aerobic exercise reduce pain perception during static continuous and pulsed heat stimulation.

Koltyn[1] did a review comparing various prescribed workloads and concluded that hypoaglesic effects were more consistent with workloads 200W and above. Hypoalgesia was also found with exercise prescribed at 65 to 75% of VO2max, but results were not conclusive when percentage of HRmax was used a prescribing criteria or subjects were allowed to select their own intensity.

Resistance Exercise/Strength Training

Very few studies have examined the relation of resistance training and pain modulation. However, a study done by Koltyn and Arbogast[8]concluded a single bout of resistance exercise can achieve a hypoalgesic response from resistance training. The resistance exercise consisted of 45 min of lifting 3 sets of 10 reps at 75% of 1RM, which included bench press, leg press, pull downs, and arm extensions.

Even though EIH has suggested to have a central response, EIH response was larger in exercising body part compared to non-exercising body part[9]. A study done by Vaegter et al[9] had their subjects perform 2 isometric contractions of dominant biceps brachii and quadriceps at 30% and 60% MVC. They concluded that high intensity isometric contraction by biceps brachii and quadriceps produced a larger local EIH compared to low intensity contraction.

An immediate local mechanical hypoaglesic response has been shown to specific exercises of cervical spine in patients having neck pain for at least 3 months[10]. Specific exercises included (1) cranio-cervical flexion with 10 second contraction for 10 repetitions with 10 second hold in between, and (2) cervical flexion endurance exercise of head lift in supine was performed for 3 sets of 10 reps at 12RM with 30 second rest in between sets (each rep lasted for 3 second with 2 second interval between reps).

A systemic review done by Hayden et al[4], concluded that supervised exercise therapy which consists of stretching and strengthening, and is individually designed improves pain and function in chronic nonspecific back pain.

Martin et al[5] designed an exercise program which has shown to be an effective management for fibromyalgia in short term. Exercise program included aerobic training, flexibility exercises and strength training.

Adverse Effects of Exercise

If exercise is considered a drug, adverse effects like any other drug follows. Exercise if not prescribed appropriately and in some cases even if prescribed appropriately may result in certain adverse effects.

It is typically accepted that exercise result in myofiber damage, and substances like lactate are release which provide nociceptive input in response to exercise. So there is a risk of flare up of symptoms after exercise. Exercise is considered a physical stressor and it has been known to activate stress responses in neuroendocrine system. And hence clinicians should be careful in prescribing exercises to patients in pain[3]. As physical stress in small amount with adequate rest-recovery period may be optimal, but excessive stress may increase pain sensitivity.

Physical Activity

Bed rest and immobility more than 2 days have never been shown beneficial, and on the contrary in geriatric population it appears to be detrimental[7]. Physical activities like Tai chi and Yoga has shown significant improvement in prevention and control of pain since ages. Ancient practice of Tai chi seems to be an effective intervention in osteoarthritis, low back pain and fibromyalgia[8]. Yoga which traces its roots 500-200 BCE is an effective adjunctive treatment for chronic low back pain as proven by Holtzman et al in a meta-analysis[10]. Regular physical activity help in preventing ill effects of immobility. It prevents joint stiffness, muscle tightness and helps in blood circulation. Other known physical activities like swimming and walking have shown to be effective in decreasing pain and improving function.

Conclusion

When prescribing exercise for pain management it is important that we:

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